1. Field of the Invention
The present invention relates to a technique of causing a single-lens reflex type digital camera to accurately capture a dark image.
2. Description of the Related Art
In a proximity shooting mode such as close-up shooting or copying using a single-lens reflex type camera, images may be blurred by impact generated as a quick return mirror moves up. To prevent this, some cameras have a mirror-up shooting function which moves the mirror up before shooting and runs the shutter curtain after impact vibrations of the mirror have decayed.
Some digital cameras have a function of accumulating charges as in final shooting without exposure of a solid-state image sensor and capturing dark image data as a noise component such as a dark current. Dark noise correction can be done by executing an arithmetic process using the dark image data and the final captured image data that is read out after charge accumulation with exposure of the image sensor.
It is therefore possible to correct image data which degrades due to dark current noise generated in the image sensor or pixel defects generated by small flaws unique to the image sensor, and obtain a high-quality image.
Especially, since dark current noise increases along with the charge accumulation time and increasing temperature of the image sensor, a large image quality improving effect can be obtained for long time exposures or exposures at a high temperature. For digital camera users, dark noise correction is a beneficial function.
In the technique disclosed in Japanese Patent Laid-Open No. 2003-264736, a dark image is captured under conditions where the dark current is expected to be large because of the shooting conditions or ambient conditions. Under other conditions, fixed pattern noise is corrected by using correction data stored in advance without capturing a dark image. This prevents degradation in image quality while minimizing dark image capturing that generates a shutter release time lag.
In the single-lens reflex type digital camera having a mirror-up shooting function, conventionally, final captured image data is acquired by final shooting with exposure while keeping the quick return mirror up. After that, dark image data is acquired by charge accumulation similar to that in final shooting without exposure. Then, dark noise correction is executed by using the final captured image data and dark image data. Conventionally, in shooting for acquiring dark image data without exposure (to be referred to as dark image capturing), the quick return mirror is kept down. For this reason, the shutter curtain and quick return mirror shield light to the image sensor so no light enters the image sensor in dark image capturing.
However, since the quick return mirror is moved up and down in every shooting even during mirror-up shooting, time is required, or the sound of mirror actuation is generated in a continuous shooting mode.
A technique that allows continuous shooting while keeping the quick return mirror up has been proposed. This technique shortens the shooting time and enables quiet shooting without any sound of mirror actuation. However, in, e.g., long-time exposure, the same time as in final shooting is necessary for capturing a dark image. If dark image capturing is executed for a long time (e.g., 1 sec or more) while keeping the quick return mirror up, light can enter from a very small gap around the closed shutter to the image sensor. This can disable accurate dark noise correction.